Assuming adiabatic expansion, and a temperature at recombination time of $2700^{\circ} C$, how much has the universe expanded if the temperature today is $2.7K$?

My idea is to use

$\frac{dQ}{dt}=A\sigma T^4$

where $\frac{dQ}{dt}=0$, A is the blackbody surface area, and T is the temperature. The way I did it was since the expansion is adiabatic (subscript $0$ for recombination time and $f$ for today):

$A_0\sigma T_0^4=A_f\sigma T_f^4$

and solving for $\frac{A_f}{A_0}$:


So the surface area of the universe is $1.47\times 10^{12}$ larger now than at recombination time. Is this correct?


closed as off-topic by JMac, G. Smith, GiorgioP, John Rennie, Kyle Kanos Apr 2 at 11:39

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  • $\begingroup$ It is more useful to think about the energy density. The energy density of a gas of photon also goes as $\propto T^4$. How does the energy density of relativistic matter depend on the scale factor? This will tell you how $T$ depends on the scale factor. $\endgroup$ – octonion Apr 1 at 18:24

The temperature $T$ of the cosmic background radiation is given by the energy of the photons, which redshift proportionally to the expansion of the Universe. That is, $$ a T = a_0 T_0, $$ where $a$ is the scale factor, and subscript $0$ denote values today. Hence, when $T$ drops to a fraction $T_0 / T = 2.7\,\mathrm{K} \,/\, 3000\,\mathrm{K} \simeq 1/1100$, that means that the Universe has expanded by a factor $a_0/a = 1100$.

Usually we define the scale factor today to be unity, and just say $a$ was $1/1100 \simeq 9\times10^{-4}$ at that time. The redshift $z$ of the photons received from that time is then $z+1=1/a\simeq1100$.

This expansion is in all directions. So distances between galaxies (that are not so close that they are gravitationally bound) increase by a factor $a^{-1}$, the surface area of the observable Universe increases by a factor $a^{-2} \simeq 1.2\times10^6$, and the volume of the Universe increases by a factor $a^{-3} \simeq 1.3\times10^9$.

Note that when you talk about how much the Universe has expanded, it is common to quote the "linear" expansion, i.e. the scale factor $a$, although in principle the volume factor $a^3$ might strictly be a more appropriate term. In contrast, I've never seen anybody talk about how much the surface area of the observable Universe has expanded; $a^2$ doesn't really bear any physical significance, but $a$ and $a^3$ does, as they describe the change in distances between objects, and volumes and — in particular — densities of objects, respectively.

  • $\begingroup$ “Usually we define the scale factor today to be unity, and just say 𝑎=1100.” The universe had a smaller scale factor in the past, not a larger one. You got your ratio backward. $\endgroup$ – G. Smith Apr 1 at 19:32
  • $\begingroup$ It is the redshift, not the scale factor, that is estimated to be about 1100. $\endgroup$ – D. Halsey Apr 1 at 21:09
  • $\begingroup$ Okay I'm sorry, I was a bit too fast here. Thanks @G.Smith $\endgroup$ – pela Apr 1 at 21:25
  • $\begingroup$ …and @D.Halsey :) $\endgroup$ – pela Apr 1 at 21:25
  • $\begingroup$ Downvoted? But why? $\endgroup$ – pela Apr 2 at 11:54

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